Mid-Pleistocene to present stratigraphic responses in a tectonically-driven depositional setting: Eel River Basin, northern California

textThe Eel River Basin of northern California is an actively deforming forearc basin, affected today by Gorda-North America Plate convergence, northward migration of the Mendocino Triple Junction, glacioeustatic sea-level fluctuations that periodically exposed the continental shelf, and high rates of sediment input. A high-resolution multichannel seismic reflection survey of the shelf and upper slope has been conducted as part of the Office of Naval Research STRATAFORM initiative, with the goal of understanding processes affecting sediment dispersal and preservation in this complex continental margin setting. Interpretations of the seismic images indicate that, although tectonism strongly overprints preserved sequence morphologies, glacioeustacy is of primary importance in the distribution and preservation of sediments along this margin. On the shelf, stacked sequences dominated by interpreted highstand marine sediment are separated by prominent ravinement surfaces formed during sea-level transgressions. Folding and faulting locally modify sequence morphologies. At the south end of the seismic grid, localized uplift attributed to Mendocino Triple Junction encroachment overrides the glacioeustatic stacking pattern dominant over the rest of the shelf, inducing shelf incision and preferential preservation of vertically stacked channel-fills. An abrupt shift in shelf sequence morphology ~500 ka suggests that the triple junction began to affect the southern basin at that time, by decreasing accommodation space marginwide. On the adjacent slope, effects of tectonism and glacioeustacy can also be discerned. In the south, the Humboldt Slide is identified as a long-lived mass-wasting feature, likely triggered by triple junction-related uplift and associated seismicity ~450-500 ka. However, a cyclical succession of contrasting lithologies is also apparent in the deformed slide sequences, which I attribute to glacioeustatic cyclicity. To the north, effects of regional tectonism are less apparent: the slope is dominated by channels and gullies that incise and infill due to presumed glacioeustatically-induced variations in slope sediment supply. However, shifts in lateral channel distributions through time may be an indication of marginal uplift. Although ground truth is needed to test the broad stratigraphic development models proposed by this study, seismic images of the Eel River Basin confirm the complex interplay of processes influencing sediment deposition and preservation in this active margin setting.Geological Science